To provide for a satisfactory performance, cold-work tool steels must attain a required hardness, possess sufficient toughness and be resistant to wear.
The wear resistance of tool steels depends on the amount, the type, and the size distribution of primary carbides, as well as the overall hardness. Primary alloy carbides, due to their very high hardness, are the main contributors to wear resistance. Among all the types of primary carbides commonly found in tool steels the vanadium-rich MC primary carbides possess the highest hardness. Niobium also forms very hard Nb-rich MC carbides but its usage in tool steels produced by ingot metallurgy has been limited due to its tendency to form large MC carbides, which has detrimental effects on the toughness of Nb-containing tool steel.
To obtain the desired combination of toughness and resistance to wear in the cold-work tool steel of the invention, it is necessary to obtain a dispersion of very small MC primary carbides uniformly distributed in a matrix of tempered martensite.
Based on thermodynamic calculations (performed with Thermo-Calc software coupled with TCFE3 thermodynamic database) it was discovered that adding niobium to a cold work-tool steel composition (produced by powder metallurgy processing) results in a larger driving force for precipitation of MC type Nb-rich primary carbides, which in turn leads to a finer distribution of primary carbides. The following nominal chemical composition (in weight percent) of a new high-toughness cold-work tool steel grade has been formulated: Fe-0.8C-7.5Cr-0.75V-2.5Nb-1.3Mo-1.5W-0.1 N. The chemical composition of the matrix of the alloy of the invention and the volume fraction of MC primary carbides in the alloy of invention are similar to those characteristics of some other selected commercially produced cold work tool steels to provide desired hardening and wear resistance characteristics. PM metallurgy steel grade (referred to as Alloy A) and a conventional metallurgy tool steel grade (referred to as Alloy B), which compositions are listed in Table 1. Both steels (Alloy A and Alloy B) are used as the benchmark cold-work tool steels for comparison of toughness and strength properties, as well as the microstructural characteristics.